Optical element

ABSTRACT

An optical element including: an alignment substrate; a liquid crystal layer formed on the alignment substrate, made by forming and curing a film of a liquid crystalline material; and a protective layer having high hardness, formed on the liquid crystal layer. The protective layer is for protecting the liquid crystal layer from being deformed by externally exerted forces. Preferably, the protective layer has a modulus of elasticity (=(elastic deformation)/(total deformation)) of 0.6 or more and a plastic deformation of 0.5 μm or less as determined by pushing an indenter into the protective layer with a test force of 2 mN in accordance with the universal hardness test method. The optical element has the advantages that the film thickness distribution of the liquid crystal layer remains uniform even when forces are externally exerted to the optical element in the process of production of the optical element or in the course of incorporation of the optical element in a liquid crystal display, and that the optical element can maintain its high displaying quality even when incorporated in a liquid crystal display.

This is a Division of U.S. patent application Ser. No. 10/629,908. Theentire disclosures of the prior applications are hereby incorporated byreference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical element, such as apolarized-light-separating element, a color filter, and a retardationfilm, for use in a liquid crystal display and the like. Moreparticularly, the present invention relates to an optical elementincluding a liquid crystal layer made from a liquid crystalline materialsuch as a cholesteric liquid crystal and a nematic liquid crystal. Theterm “liquid crystal layer” as used herein refers to a layer having thenature of liquid crystal in an optical sense, and encompasses a layer inthe solidified state that a liquid crystal phase is solidified with itsmolecular orientation maintained as it is.

2. Description of Related Art

In general, a conventional liquid crystal display is composed of: aliquid crystal cell that changes, in each pixel, the state ofpolarization of illumination light emitted by an illuminator (lightsource); a pair of polarizers that are arranged on the illuminationlight incident and emergent sides of the liquid crystal cell so that theliquid crystal cell is sandwiched between them; color filters (red,green and blue) that are provided on the individual pixels of the liquidcrystal cell; and a retardation film (optical compensation sheet) forimproving the visibility of the liquid crystal cell.

In the above-described conventional liquid crystal display, theillumination light emitted by the illuminator is usually non-polarizedlight, and when this light passes through the polarizer arranged on theillumination light incident side of the liquid crystal cell, 50% or moreof the light is absorbed by the polarizer. Further, the illuminationlight emitted by the illuminator is usually white light, and when thislight passes through the color filters (red, green and blue) provided onthe individual pixels of the liquid crystal cell, 70% or more of thelight is absorbed by the color filters. Namely, in a conventional liquidcrystal display, illumination light emitted by an illuminator is mostlyabsorbed before it goes out from the viewing side of the display. Thus,the optical efficiency of a conventional liquid crystal display has beennot necessarily high.

An illuminator with high output is therefore required for a conventionalliquid crystal display of the above-described type to attain a displaywith sufficiently high brightness. However, the use of such anilluminator leads to a great increase in power consumption.

Under these circumstances, there has been proposed a method forefficiently utilizing light such as illumination light by the use of anoptical element comprising a liquid crystal layer, such as apolarized-light-separating element or a color filter, wherein theoptical element selectively transmits a part of light while reflecting apart of the reminder, and the reflected light is reused with the aid ofa reflector or the like. Specifically, Japanese Patent Publication No.2,509,372, for example, proposes a method for efficiently separatingillumination light (non-polarized light) emitted by an illuminator toobtain specific polarized light. In this method, apolarized-light-separating element including a cholesteric liquidcrystal layer is used in combination with a reflector capable ofreflecting, while inverting the direction of rotation of light(circularly polarized light) that has been reflected by thepolarized-light-separating element.

In addition, an attempt has been proposed in which a retardation filmfor eliminating a viewing-angle dependency is realized by utilizing anematic liquid crystal layer having nematic regularity or a cholestericliquid crystal layer having cholesteric regularity. Not only a λ/4retardation film of a general band, but also a λ/4 retardation film of abroadband have been proposed for such a retardation film.

However, the part of the above-described optical element that functionsoptically is a liquid crystal layer made from a liquid crystallinematerial, so that this part usually has extremely low hardness even whenthe liquid crystal layer is in the solid phase state. Therefore, ifforces are externally exerted to the liquid crystal layer in the processof production of the optical element or in the course of incorporationof the optical element in a liquid crystal display, there is thepossibility that the liquid crystal layer has dents or the like on itssurface and cannot maintain its uniform film thickness distribution. Inthe case where the liquid crystal layer in the above-described opticalelement has no uniform film thickness distribution, the state ofpolarization of light that comes out from the optical element is notuniform. If such an optical element is incorporated in a liquid crystaldisplay, it is inevitable that the liquid crystal display hasconsiderably lowered displaying quality. Further, if the above-describedoptical element is used as a retardation film to be incorporated in aliquid crystal cell of a liquid crystal display, a film thickness of theliquid crystal layer is varied by a spacer for holding a gap in theliquid crystal cell. As a result, there is generated a problem in that adesired retardation amount cannot be obtained.

SUMMARY OF THE INVENTION

The present invention was accomplished in the light of theaforementioned drawbacks in the related art. An object of the presentinvention is to provide a high-quality optical element including aliquid crystal layer, having the advantages that the film thicknessdistribution of the liquid crystal layer remains uniform even whenforces are externally exerted to the optical element in the process ofproduction of the optical element or in the course of incorporation ofthe optical element in a liquid crystal display, and that the opticalelement can maintain its high displaying quality even when incorporatedin a liquid crystal display.

The present invention provides an optical element comprising: a liquidcrystal layer made by forming and curing a film of a liquid crystallinematerial; and a protective layer formed on the liquid crystal layer, theprotective layer having hardness high enough to prevent the liquidcrystal layer from being deformed by externally exerted forces.

In the present invention, the protective layer preferably has a modulusof elasticity (=(elastic deformation)/(total deformation)) of 0.6 ormore as determined by pushing an indenter into the protective layer witha test force of 2 mN in accordance with the universal hardness testmethod. It is also preferable that the protective layer be made from amaterial comprising a resin and a monomer. Further, it is preferablethat the liquid crystalline material from which the liquid crystal layeris made has cholesteric regularity or nematic regularity. Furthermore,in the present invention, it is preferable that the optical elementfurther comprises an alignment substrate that supports the liquidcrystal layer, the alignment substrate being disposed on the surface ofthe liquid crystal layer opposite to the surface of the protectivelayer.

Furthermore, in the present invention, it is preferable that at least apart of the outer peripheral region of the liquid crystal layer isremoved, and that the protective layer is formed to cover the uppersurface as well as at least a part of the side surface of the liquidcrystal layer formed on the alignment substrate.

Furthermore, in the present invention, it is preferable that the liquidcrystal layer formed on the alignment substrate includes a plurality ofregions corresponding to display regions of the respective colors ofred, green and blue, the regions being formed with spaces therebetween,and that the protective layer is formed to cover the upper surface ofthe liquid crystal layer and to fill the spaces between the respectiveregions of the liquid crystal layer.

Furthermore, in the present invention, it is preferable that the opticalelement further comprises an alignment film and an electrode disposed onthe surface of the protective layer opposite to the surface of theliquid crystal layer, the alignment film and the electrode aligning anddriving liquid crystals in a liquid crystal cell, respectively.

Furthermore, in the present invention, it is preferable that the opticalelement further comprises a color filter layer of a light absorptiontype disposed between the liquid crystal layer and the protective layer,or disposed on the surface of the protective layer opposite to thesurface of the liquid crystal layer.

In the present invention, it is preferable that the liquid crystal layerfunctions as at least an element selected from the group consisting of apolarized-light-separating element, a color filter and a retardationfilm.

In the present invention, on top of the liquid crystal layer made byforming and curing a film of a liquid crystalline material is formed theprotective layer having hardness high enough to prevent the liquidcrystal layer from being deformed by externally exerted forces.Therefore, the film thickness distribution of the liquid crystal layerremains uniform even when forces are externally exerted to the opticalelement in the process of production of the optical element or in thecourse of incorporation of the optical element in a liquid crystaldisplay. The optical element can thus maintain its high displayingquality even when incorporated in a liquid crystal display.

Further, according to the present invention, at least a part of theouter peripheral region of the liquid crystal layer is removed, and theprotective layer is formed to cover the upper surface as well as atleast a part of the side surface of the liquid crystal layer formed onthe alignment substrate. Thus, even when incorporated in a liquidcrystal display, a sealed portion of the liquid crystal cell isprevented from interfering with the liquid crystal layer. In addition,since the protective layer is formed to cover at least a part of theside surface of the liquid crystal layer, a deterioration of the liquidcrystal layer caused by a solution from outside can effectively beprevented.

Furthermore, according to the present invention, a plurality of regionscorresponding to display regions of the respective colors of red, greenand blue, the regions being formed with spaces therebetween, areprovided to the liquid crystal layer formed on the alignment substrate.In addition, the protective layer is formed to cover the upper surfaceof the liquid crystal layer and to fill the spaces between therespective layers of the liquid crystal layer. Since each part of theprotective layer, which fills the spaces between the respective regionsof the liquid crystal layer, functions like a column, the liquid crystallayer can more efficiently be prevented from being deformed byexternally exerted forces.

Furthermore, according to the present invention, a color filter layer ofa light absorption type is disposed between the liquid crystal layer andthe protective layer, or disposed on the surface of the protective layeropposite to the surface of the liquid crystal layer. Thus, both of theprotective layer and the liquid crystal layer can be protected by thecolor filter layer of a light absorption type having high hardness, andtherefore the liquid crystal layer can more efficiently be preventedfrom being deformed by externally exerted forces.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings,

FIG. 1 is a diagrammatic cross-sectional view for illustrating anembodiment of the optical element according to the present invention;

FIG. 2 is a flow chart of the process for producing the optical elementshown in FIG. 1; and

FIGS. 3A and 3B are schematic cross-sectional views showing a firstmodification of the optical element shown in FIG. 1;

FIG. 4 is a schematic cross-sectional view showing a second modificationof the optical element shown in FIG. 1;

FIG. 5 is a schematic cross-sectional view showing a third modificationof the optical element shown in FIG. 1;

FIGS. 6A and 6B are schematic cross-sectional views showing a fourthmodification of the optical element shown in FIG. 1;

FIGS. 7A and 7B are schematic cross-sectional views showing a fifthmodification of the optical element shown in FIG. 1;

FIGS. 8A and 8B are schematic cross-sectional views showing a sixthmodification of the optical element shown in FIG. 1; and

FIG. 9 is a chart for illustrating the hardness (modulus of elasticity)of the optical element shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

By referring to the accompanying drawings, embodiments of the presentinvention will be described hereinafter.

The whole structure of an optical element according to this embodimentis firstly described with reference to FIG. 1.

As shown in FIG. 1, an optical element 10 is composed of an alignmentsubstrate 11, a liquid crystal layer 12 provided on the alignmentsubstrate 11, made by forming and curing a film of a liquid crystallinematerial, and a protective layer 13 having high hardness, formed on theliquid crystal layer 12.

The alignment substrate 11 is for supporting the liquid crystal layer12, and also for aligning liquid crystalline molecules in the liquidcrystal layer 12. This alignment substrate 11 is in contact with thesurface of the liquid crystal layer 12 opposite to the surface that isin contact with the protective layer 13. Examples of materials that canbe used for the alignment substrate 11 include glass substrates on whichfilms of alignment materials such as polyimide are formed and thenrubbed, glass substrates on which films of polymeric compounds thatserve as optical alignment layers are formed and then irradiated withpolarized UV (ultraviolet light), and oriented PET (polyethyleneterephthalate) films.

The liquid crystal layer 12 is made from a liquid crystalline materialhaving cholesteric regularity and has the polarized-light-selectingproperty (polarized-light-separating property) of separating a componentpolarized (circularly polarized) in one direction from a componentpolarized in the opposite direction according to the physicalorientation (planar orientation) of liquid crystalline molecules in theliquid crystal layer 12. Thanks to this property, the optical element 10including the liquid crystal layer 12 can function as apolarized-light-separating element, a color filter, or a retardationfilm. For the liquid crystal layer 12, there can be used liquidcrystalline molecules (liquid crystalline monomers or oligomers) thatare polymerized when irradiated with ultraviolet light, electron beams,or the like, as well as liquid crystalline polymers.

When using, for example, photopolymerizable liquid crystalline monomers,a chiral nematic liquid crystal (cholesteric liquid crystal) is obtainedby adding a chiral agent to liquid crystalline monomers (nematic liquidcrystal) presenting a nematic liquid crystal phase.

More specifically, a nematic liquid crystal preferably has two or morepolymerizable functional groups. For example, liquid crystallinemonomers shown in the below chemical formulas (1) to (10) can be used toform the liquid crystal layer 12. In the chemical formulas (1) to (10),“X” preferably represents 2 to 6 (integral number).

A chiral agent preferably has a polymerizable functional group at itsterminal. For example, chiral agents shown in the below chemicalformulas (11) to (14) can be used. Alternately, chiral agents disclosedin Japanese Patent Laid-Open Publications Nos. 2000-95883, 245960/1996and 53074/1997 can also be used to form the liquid crystal layer 12. Inthe chemical formulas (11) to (14), “X” preferably represents 2 to 6(integral number).

The protective layer 13 is for preventing the liquid crystal layer 12from being deformed by externally exerted forces. Examples of materialsthat can be used for the protective layer 13 include resins such asacrylic resins and urethane resins, and mixtures of two or more monomersselected from acrylic monomers, urethane monomers, and so forth.

Preferably, the protective layer 13 has a modulus of elasticity(=(elastic deformation)/(total deformation)) of 0.6 or more and aplastic deformation of 0.5 μm or less as determined by pushing anindenter into the protective layer 13 with a test force of 2 mN inaccordance with the universal hardness test method. Further, it ispreferable that the optical element 10 as shown in FIG. 1 (the wholebody of the protective layer 13, the liquid crystal layer 12 and thealignment substrate 11) has a modulus of elasticity of 0.6 or more and aplastic deformation of 0.5 μm or less as determined by pushing, from theprotective layer 13 side, an indenter into the liquid crystal layer 12with a test force of 2 mN in accordance with the universal hardness testmethod.

The universal hardness test method is that an indenter is pushed into atest object and the depth of indenter penetration (i.e., deformation) ismeasured (standard: DIN 50359). In this method, when an indenter ispushed into a test object with a predetermined force (2 mN in the caseshown in FIG. 9), the test object is deformed, drawing a hysteresiscurve (indentation force versus deformation) as shown in FIG. 9. Asshown in this figure, after the test object is deformed from initialpoint O₁, at which the indentation force is 0, to midpoint O₂, at whichthe indentation force is 2 mN, the indentation force is maintained 2 mNfor a certain period of retention time during which the deformation ofthe test object changes from midpoint O₂ to midpoint O₃. The indentationforce is then released. As a result, the deformation of the test objectreturns to final point O₄. If the test object is a perfectly elasticbody, the deformation at final point O₄ is zero. Actually, however,there is no such case that the test object is a perfectly elastic body,so that the deformation of the test object at final point O₄ is apositive value. This value is equivalent to the plastic deformation.When the deformation at the point of time at which the indentation ofthe indenter is completed (midpoint O₃) is taken as the totaldeformation, the value obtained by subtracting the above-describedplastic deformation from the total deformation is equal to the elasticdeformation. In general, it is possible to define, by the use of thesedeformations, the modulus of elasticity as follows: (modulus ofelasticity)=(elastic deformation)/(total deformation). The modulus ofelasticity can be used as a measure of the hardness of a test object.Namely, a test object having a smaller plastic deformation and a greatermodulus of elasticity is harder; on the contrary, a test object having agreater plastic deformation and a smaller modulus of elasticity issofter.

Although the hardness of the protective layer 13 and that of the wholeoptical element 10 are herein determined by the universal hardness testmethod, they may also be determined by the pencil hardness test method,the Vickers hardness test method, or the like.

Next, a process for producing the optical element 10 shown in FIG. 1 isdescribed with reference to FIG. 2. Taken herein as an example todescribe the process is a case where a cholesteric liquid crystallinemonomer that is polymerized when irradiated with ultraviolet light isused to form the liquid crystal layer 12.

First of all, a cholesteric liquid crystalline monomer solutioncontaining a photopolymerization initiator is prepared. This solution isapplied to an alignment substrate 11 and is then dried to form a liquidcrystal layer 12′ in the uncured state (FIG. 2( a)).

Ultraviolet light with a predetermined amount of energy is then appliedto the uncured liquid crystal layer 12′ in a predetermined atmosphere toobtain a liquid crystal layer 12 in the cured state (FIG. 2( b)).

Thereafter, the cured liquid crystal layer 12 is heated, as needed, at apredetermined temperature for baking (FIG. 2( c)), and is then coatedwith a material for forming a protective layer 13 (FIG. 2( d)). Thus,the optical element 10 is finally produced.

In this embodiment, the protective layer 13 having high hardness isformed on the liquid crystal layer 12 made by forming and curing a filmof a liquid crystalline material to protect the liquid crystal layer 12from being deformed by externally exerted forces. Therefore, the filmthickness distribution of the liquid crystal layer 12 remains uniformeven when forces are externally exerted to the optical element 10 in theprocess of production of the optical element 10 or in the course ofincorporation of the optical element 10 in a liquid crystal display. Theoptical element 10 can thus maintain its high displaying quality evenwhen incorporated in a liquid crystal display.

In the above embodiment, a case where the liquid crystal layer 12 isformed of a liquid crystalline material having cholesteric regularity istaken as an example. However, not limited thereto, a case where theliquid crystal layer 12 is formed of a liquid crystalline materialhaving nematic regularity may also be applied to the present invention.The liquid crystal 12 formed of a liquid crystalline material havingnematic regularity can suitably fulfill a function as a retardationfilm.

In the above embodiment, a case where the liquid crystal layer 12 isformed over the whole surface of the alignment substrate 11 is taken asan example. However, not limited thereto, the crystal liquid layer 12formed on the alignment substrate 11 may be suitably patterned.

Specifically, as shown in FIGS. 3A and 3B, it is preferable that theliquid crystal layer 12 formed on the alignment substrate 11 ispatterned such that the outer peripheral region of the liquid crystallayer 12 is removed, and that the protective layer 13 is formed to coverthe upper surface as well as the side surface of the liquid crystallayer 12 formed on the alignment substrate 11. Thus, even whenincorporated in a liquid crystal display, a sealed portion of the liquidcrystal cell is prevented from interfering with the liquid crystal layer12. In addition, since the protective layer 13 is formed to cover theside surface of the liquid crystal layer 12, a deterioration of theliquid crystal layer 12 caused by a solution from outside caneffectively be prevented. In FIG. 3A, only the outer periphery of theliquid crystal layer 12 is removed, and the outer periphery of theprotective layer 13 is not removed. However, not limited thereto, asshown in FIG. 3B, the outer periphery of the protective layer 13 mayalso be removed so as to correspond to the pattern of the liquid crystallayer 12. In FIGS. 3B and 3( b), the alignment substrate 11 is composedof a support substrate 11 a such as a glass substrate, and an alignmentfilm 11 b formed on the support substrate 11 a and corresponding to thepattern (pattern having the outer periphery being removed) of the liquidcrystal layer 12.

As shown in FIG. 4, it is also preferable that the liquid crystal layer12 formed on the alignment substrate 11 is patterned such that aplurality of regions 12R, 12G and 12B correspond to display regions ofthe respective colors of red, green, and blue, the regions being formedwith spaces therebetween. It is also preferable that the protectivelayer 13 is formed to cover the upper surface of the liquid crystallayer 12 and to fill the spaces between the respective regions 12R, 12G,and 12B of the liquid crystal layer 12. Since each part of theprotective layer 13, which fills the spaces between the respectiveregions 12R, 12G and 12B of the liquid crystal layer 12, functions likea column, the liquid crystal layer 12 can more efficiently be preventedfrom being deformed by externally exerted forces. In FIG. 4, thealignment substrate 11 is composed of the support substrate 11 a such asa glass substrate, and the alignment film 11 b formed on the supportsubstrate 11 a and corresponding to the pattern (pattern having theouter periphery being removed) of the liquid crystal layer 12. FIG. 4shows the regions 12R, 12G and 12B individually corresponding to theregions of the respective pixels of red, green and blue. However, notlimited thereto, the respective regions 12R, 12G and 12B may be dividedinto two or more regions with spaces therebetween.

As shown in FIG. 5, in the optical element 10 shown in FIG. 3B, analignment film 14 and a transparent electrode 15 such as an ITO film maybe disposed on the surface of the protective layer 13 opposite to thesurface of the liquid crystal layer 12, the alignment film 14 and thetransparent electrode 15 aligning and driving liquid crystals in aliquid crystal cell, respectively. In FIG. 5, although the opticalelement 10 shown in FIG. 3B is taken as an example, the alignment film14 and the transparent electrode 15 may be disposed similarly to theoptical element 10 shown in FIGS. 1, 3A and 4. In FIG. 5, the alignmentsubstrate 11 is composed of the support substrate 11 a such as a glasssubstrate, and the alignment film 11 b formed on the support substrate11 a and corresponding to the pattern (pattern having the outerperiphery being removed) of the liquid crystal layer 12.

As shown in FIGS. 6A and 6B, a color filter layer 14 of a lightabsorption type (pigment dispersion type) including a plurality ofcolored regions 14R, 14G and 14B may be disposed between the liquidcrystal layer 12 and protective layer 13, or may be disposed on thesurface of the protective layer 13 opposite to the surface of the liquidcrystal layer 12. The plurality of colored regions 14R, 14G, and 14Bcorrespond to display regions of the respective colors of red, green andblue. Thus, both of the protective layer 13 and the liquid crystal layer12 can be protected by the color filter layer 14 of a light absorptiontype having high hardness, and therefore the liquid crystal layer 12 canmore efficiently be prevented from being deformed by externally exertedforces. In FIGS. 6A and 6B, the alignment substrate 11 is composed ofthe support substrate 11 a such as a glass substrate and the alignmentfilm 11 b formed on the support substrate 11 a. FIGS. 6A and 6B show ablack matrix 15 made of chrome or resin, the black matrix 15 beingformed on each region between the plurality of colored regions 14R, 14G,and 14B adjacent to each other. In FIGS. 6A and 6B, the black matrix 15is formed in contact with the respective colored regions 14R, 14G and14B of the color filter layer 14. Alternately, as shown in FIGS. 7A and7B, the black matrix 15 may be apart from the respective colored regions14R, 14G and 14B of the color filter layer 14, and formed on the supportsubstrate 11 a of the alignment substrate 11.

FIGS. 6A, 6B, 7A and 7B show the color filter layer 14 of a lightabsorption type being disposed between the liquid crystal layer 12 andthe protective layer 13, or disposed on the surface of the protectivelayer 13 opposite to the surface of the liquid crystal layer 12.However, not limited thereto, the color filter layer 14 of a lightabsorption type may be disposed under the liquid crystal layer 12, asshown in FIGS. 8A and 8B. In this modification, the liquid crystal layer12 can be more tightly in contact with the alignment substrate 11, sothat an improved displaying quality can be obtained when incorporated ina liquid crystal display.

EXAMPLE

The present invention will now be explained more specifically byreferring to the following example, which is not intended to limit orrestrict the scope of the invention in any way.

Example 1

A polyimide film (LX1400 (manufactured by Hitachi Chemical Co., Ltd.,Japan)) with a thickness of 0.02 μm was formed on a glass substrate.After baked at 250° C., the polyimide film was then subjected to rubbingtreatment for alignment.

The rubbed polyimide film on the glass substrate was spin-coated with asolution containing a cholesteric liquid crystal having a composition asdescribed below.

Nematic liquid crystal (above formula (8)): 95.45% by weight

Chiral agent (above formula (14)): 4.55% by weight

Polymerization initiator (Irg907): 5% by weight Surface active agent(below formula (15)): 0.05% by weight

Toluene: 175% by weight

As described above, the cholesteric liquid crystal solution contains acholesteric liquid crystal (chiral nematic liquid crystal) which is amixture of a nematic liquid crystal and a chiral agent.

The coating film applied to the polyimide film on the glass substratewas subjected to a treatment for alignment by drying on a hot plate at80° C. for 1 minute. Then, the coating film was visually observed thatthe film presented a cholesteric phase.

Next, the coating film is irradiated with ultraviolet light (20 mW/cm²,365 nm) by using a ultra-high-pressure mercury lamp for 10 seconds toobtain a cholesteric liquid crystal layer. The thickness of thecholesteric liquid crystal layer was made 3.0 μm to make the selectivereflectance of the cholesteric liquid crystal layer 100%. From themeasurement using a spectrophotometer, it was found that the selectivereflection wave range of the cholesteric liquid crystal layer wascentered at 515 nm.

JNPC-80 (manufactured by JSR Corporation, Japan), a material for forminga protective layer, was applied by spin coating to the cholestericliquid crystal layer formed in the above-described manner, and was driedon a hot plate at 90° C. for 1 minute. After that, the coating film isirradiated with ultraviolet light (20 mW/cm², 365 nm) to expose it byusing a ultra-high pressure mercury lamp for 20 seconds, and then theirradiated coating film was baked for 1 hour to finally form aprotective layer. The thickness of the protective layer was made 2.0 μm.Separately, a protective layer was formed directly on a glass substratein the same manner, and its modulus of elasticity was determined bypushing an indenter into the protective layer with a test force of 2 mNin accordance with the universal hardness test method. As a result, themodulus of elasticity was found to be 0.62.

There was thus finally obtained an optical element composed of thecholesteric liquid crystal layer having thereon the protective layer.The modulus of elasticity of this optical element according to Example 1was determined by pushing, from the protective layer side, an indenterinto the cholesteric liquid crystal layer with a test force of 2 mN inaccordance with the universal hardness test method. As a result, themodulus of elasticity was 0.65, and the plastic deformation was 0.45 μm.

Example 2

An optical element was produced in the same manner as that of Example 1,other than that the thickness of the protective layer was made 1.5 μm.The modulus of elasticity of thus obtained optical element according toExample 2 was determined by pushing, from the protective layer side, anindenter into the cholesteric liquid crystal layer with a test force of2 mN in accordance with the universal hardness test method. As a result,the modulus of elasticity was 0.60, and the plastic deformation was 0.48μm. A protective layer was formed directly on a glass substrate in thesame manner as that of Example 1, and its modulus of elasticity wasdetermined by pushing an indenter into the protective layer with a testforce of 2 mN in accordance with the universal hardness test method. Asa result, the modulus of elasticity was found to be 0.60.

Example 3

An optical element was produced in the same manner as that of Example 1,other than that JSS-341 (manufactured by JSR Corporation) was used as amaterial for forming a protective layer, with its thickness being 1.5μm, and that the protective layer was not subjected to exposure processwhen forming the protective layer. The modulus of elasticity of thusobtained optical element according to Example 3 was determined bypushing, from the protective layer side, an indenter into thecholesteric liquid crystal layer with a test force of 2 mN in accordancewith the universal hardness test method. As a result, the modulus ofelasticity was 0.65, and the plastic deformation was 0.46 μm. Aprotective layer was formed directly on a glass substrate in the samemanner as that of Example 1, and its modulus of elasticity wasdetermined by pushing an indenter into the protective layer with a testforce of 2 mN in accordance with the universal hardness test method. Asa result, the modulus of elasticity was found to be 0.64.

Example 4

An optical element was produced in the same manner as that of Example 1,other than that an ITO layer (1500 Å thickness) as a transparentelectrode was formed on a surface of a protective layer by spattering,and then a polyimide film (LX1400 (manufactured by Hitachi Chemical Co.,Ltd., Japan); 0.07 μm thickness) as an alignment film was formed on theITO layer. The polyimide film on the ITO layer was formed in the samemanner as the alignment film on a glass substrate in Example 1. Themodulus of elasticity of thus obtained optical element according toExample 4 was determined by pushing, from the protective layer side, anindenter into the cholesteric liquid crystal layer with a test force of2 mN in accordance with the universal hardness test method. As a result,the modulus of elasticity was 0.66, and the plastic deformation was 0.46μm. A protective layer was formed directly on a glass substrate in thesame manner as that of Example 1, and its modulus of elasticity wasdetermined by pushing an indenter into the protective layer with a testforce of 2 mN in accordance with the universal hardness test method. Asa result, the modulus of elasticity was found to be 0.62.

Example 5

An optical element was produced in the same manner as that of Example 4,other than that a color filter layer of a light absorption type (pigmentdispersion type) was formed between a liquid crystal layer and aprotective layer by a photolithography method. The modulus of elasticityof thus obtained optical element according to Example 5 was determinedby pushing, from the protective layer side, an indenter into thecholesteric liquid crystal layer with a test force of 2 mN in accordancewith the universal hardness test method. As a result, the modulus ofelasticity was 0.67, and the plastic deformation was 0.53 μm. Aprotective layer was formed directly on a glass substrate in the samemanner as that of Example 1, and its modulus of elasticity wasdetermined by pushing an indenter into the protective layer with a testforce of 2 mN in accordance with the universal hardness test method. Asa result, the modulus of elasticity was found to be 0.62.

Example 6

An optical element was produced in the same manner as that of Example 4,other than that a color filter layer of a light absorption type (pigmentdispersion type) was formed between a glass substrate and a polyimidefilm by a photolithography method. The modulus of elasticity of thusobtained optical element according to Example 6 was determined bypushing, from the protective layer side, an indenter into thecholesteric liquid crystal layer with a test force of 2 mN in accordancewith the universal hardness test method. As a result, the modulus ofelasticity was 0.64, and the plastic deformation was 0.57 μm. Aprotective layer was formed directly on a glass substrate in the samemanner as that of Example 1, and its modulus of elasticity wasdetermined by pushing an indenter into the protective layer with a testforce of 2 mN in accordance with the universal hardness test method. Asa result, the modulus of elasticity was found to be 0.62.

Example 7

An optical element was produced in the same manner as that of Example 1,other than that an SiO₂ layer (0.3 μm thickness) as another protectivelayer was formed on a surface of the formed protective layer byspattering. The modulus of elasticity of thus obtained optical elementaccording to Example 7 was determined by pushing, from the laminatedprotective layer side, an indenter into the cholesteric liquid crystallayer with a test force of 2 mN in accordance with the universalhardness test method. As a result, the modulus of elasticity was 0.67,and the plastic deformation was 0.54 μm. A laminated protective layer(JNPC-80 layer and SiO₂ layer) was formed directly on a glass substratein the same manner as that of Example 1, and its modulus of elasticitywas determined by pushing an indenter into the laminated protectivelayer with a test force of 2 mN in accordance with the universalhardness test method. As a result, the modulus of elasticity was foundto be 0.66.

Comparative Example 1

An optical element was produced in the same manner as that of Example 1,other than that no protective layer was formed. The modulus ofelasticity of thus obtained optical element according to ComparativeExample 1 was determined by pushing, from the protective layer side, anindenter into the cholesteric liquid crystal layer with a test force of2 mN in accordance with the universal hardness test method. As a result,the modulus of elasticity was 0.54, and the plastic deformation was 0.72μm.

(Evaluation Result)

After producing each optical element according to Examples 1 to 7 in anactual production line, it was incorporated in a liquid crystal display.In this process, the film thickness distribution of the liquid crystallayer remained uniform, and the displaying quality of the displayfinally obtained was found excellent. On the other hand, after producingan optical element according to Comparative Example 1 in an actualproduction line, it was incorporated in a liquid crystal display. Inthis process, the liquid crystal layer could not remain its uniform filmthickness distribution, and the displaying quality of the displayfinally obtained was lowered.

Thus, according to the present invention, the film thicknessdistribution of the liquid crystal layer remains uniform even whenforces are externally exerted to the optical element in the process ofproduction of the optical element or in the course of incorporation ofthe optical element in a liquid crystal display, and the optical elementcan maintain its high displaying quality even when incorporated in aliquid crystal display.

1. An optical element comprising: a liquid crystal layer made by formingand curing a film of a liquid crystalline material, the liquid crystallayer including a liquid crystal phase in a solidified state such that amolecular orientation of the liquid crystal phase is maintained evenwhen an electrical force is applied; and a protective layer formed onthe liquid crystal layer, the protective layer having a hardnesssufficient to prevent the liquid crystal layer from being deformed byexternally exerted forces; wherein the optical element further comprisesa color filter layer of a light absorption type disposed on a surface ofthe liquid crystal layer opposite from the protective layer, and whereinthe liquid crystalline material from which the liquid crystal layer ismade has cholesteric regularity.
 2. The optical element according toclaim 1, wherein the protective layer has a modulus of elasticity(=(elastic deformation)/(total deformation)) of 0.6 or more asdetermined by pushing an indenter into the protective layer with a testforce of 2 mN in accordance with the universal hardness test method. 3.The optical element according to claim 1, wherein the protective layeris made from a material that comprises a resin and a monomer.
 4. Theoptical element according to claim 1, further comprising an alignmentsubstrate that supports the liquid crystal layer, the alignmentsubstrate being disposed on a surface of the liquid crystal layeropposite from the protective layer.
 5. An optical element comprising: aliquid crystal layer made by forming and curing a film of a liquidcrystalline material, the liquid crystal layer including a liquidcrystal phase in a solidified state such that a molecular orientation ofthe liquid crystal phase is maintained even when an electrical force isapplied; and a protective layer formed on the liquid crystal layer, theprotective layer having a hardness sufficient to prevent the liquidcrystal layer from being deformed by externally exerted forces; whereinthe optical element further comprises a color filter layer of a lightabsorption type disposed on a surface of the liquid crystal layeropposite from the protective layer, and wherein the liquid crystallinematerial from which the liquid crystal layer is made has nematicregularity.
 6. The optical element according to claim 5, furthercomprising an alignment substrate that supports the liquid crystallayer, the alignment substrate being disposed on a surface of the liquidcrystal layer opposite from the protective layer.
 7. The optical elementaccording to claim 5, wherein the protective layer has a modulus ofelasticity (=(elastic deformation)/(total deformation)) of 0.6 or moreas determined by pushing an indenter into the protective layer with atest force of 2 mN in accordance with the universal hardness testmethod.
 8. The optical element according to claim 5, wherein theprotective layer is made from a material that comprises a resin and amonomer.